1. Field of the Invention
The present invention relates to an ultrasonic-wave propagation-time measuring method, a gas-pressure measuring method, a gas-flow-rate measuring method, and a gas sensor.
2. Description of the Related Art
Techniques for measuring sound velocity on the basis of propagation time of ultrasonic waves have been proposed in, for example, Japanese Patent Publication (kokoku) No. 5-72527 and Japanese Patent Application Laid-Open No. 60-502171.
In these techniques, an ultrasonic element is caused to transmit an ultrasonic wave (transmission wave) and receive its reflection wave (reception wave); and sound velocity is measured on the basis of the propagation time between transmission of the transmission wave and reception of the reception wave.
In a well-known method of measuring propagation time; more particularly, with regard to detecting the reception signal, a threshold level (reference value) of a comparator is fixed, and the reception wave itself or an integrated value of the reception wave is compared with the fixed threshold level.
However, ultrasonic reception waves sometimes attenuate due to pressure or other causes. In such a case, since the conventional techniques use a fixed value for the threshold level of the comparator, a measured propagation time includes an error stemming from attenuation of the reception wave.
Thus, in some cases a sound velocity calculated from the propagation time becomes inaccurate. Therefore, a gas concentration sensor which detects gas concentration on the basis of sound velocity encounters difficulty in accurately detecting gas concentration.
The present invention has been achieved in order to solve the above problems, and an object of the invention is to provide an ultrasonic-wave propagation-time measuring method which enables accurate determination of propagation time, a gas-pressure measuring method, a gas-flow-rate measuring method, and a gas sensor.
The above objectives of the invention are achieved by providing:
(1) An ultrasonic-wave propagation-time measuring method in which an ultrasonic wave is transmitted by use of an ultrasonic element, a reflection wave of the transmission wave is received as a reception wave by use of the same ultrasonic element or a different ultrasonic element, and a period of time between transmission of the ultrasonic wave and reception of the reception wave is measured as a propagation time, the method being characterized by comprising: a reference-value setting step of integrating a reception wave or a portion thereof in order to obtain an integral value, and setting a reference value on the basis of the integrated value; and a time measurement step performed when a propagation time is to be measured and comprising integrating a reception wave or a portion thereof in order to obtain an integral value, and measuring, as an arrival time of the reception wave, a point in time when the integral value attains the reference value.
In the present invention, an ultrasonic reception wave or a portion thereof is integrated in order to obtain an integral value, and a reference value (a threshold level) is set on the basis of the integrated value. At the time of actual measurement of propagation time, a reception wave or a portion thereof is integrated in order to obtain an integral value, and a point in time when the integral value attains the reference value is measured as an arrival time of the reception wave.
That is, ultrasonic reception waves sometimes attenuate due to the surrounding atmosphere (e.g., atmospheric pressure). In such a case, when a fixed reference value is used as in the conventional techniques, the time at which an integral value of a reception wave attains the reference value varies depending on the surrounding atmosphere, so that accurate measurement of propagation time becomes impossible. Therefore, in the present invention, the reference value is adjustably set on the basis of an actual integral value.
Accordingly, when the integral value of a reception wave decreases due to attenuation of the reception wave, the reference value (which is set on the basis of the integral value) also decreases. Therefore, at the time of actual measurement, the point in time when the integral value of a reception wave attains the (lowered) reference value represents an accurate arrival time. In other words, in the present invention, the reference value itself can be adjusted in consideration of pressure and other factors which influential propagation time. Therefore, even when a reception wave is attenuated due to pressure or other causes during actual measurement, the propagation time between transmission of the transmission wave and reception of the reception wave does not change, so that accurate measurement of propagation time can be effected at all times.
(2) The ultrasonic-wave propagation-time measuring method as described in (1), which further comprises integrating the reception wave or a portion thereof in the reference-value setting step, and setting a predetermined integral value obtained during a period between the start of integration and the time when a maximum integral value is obtained as the reference value.
In embodiment (2), a reference value which is optimal for measurement of propagation time can be set freely within the range in which the integral value of the reception wave varies. For example, in order to set the reference value, a peak integral value (voltage value) which is held using a peak-hold circuit is divided by use of a resistance-voltage-division circuit.
(3) The ultrasonic-wave propagation-time measuring method as described in (2), which further comprises setting the reference value to half the maximum integral value.
In embodiment (3) above, a value corresponding to half the maximum integral value (a center point of the range of variation of the integral value) is used as the reference value. Since the reference value corresponds to the center of the range of variation of the integral value, measurement of propagation time is hardly affected by noise and other causes, and failure to judge arrival time accurately can be avoided. Therefore, the present invention is practically useful.
(4) The ultrasonic-wave propagation-time measuring method as described in (1), which comprises representing the integral value in the form of a voltage valve in the reference-value setting step, and setting a ratio of the reference value to a maximum integral value on the basis of a maximum voltage value, which represents the maximum integral value.
In embodiment (4) above, for example, a peak integral value (maximum voltage value) which is held by use of a peak-hold circuit is reduced at a predetermined ratio by use of a voltage amplifying circuit in order to set the reference value.
(5) An ultrasonic-wave propagation-time measuring method in which an ultrasonic wave is transmitted by use of an ultrasonic element, a reflection wave of the transmission wave is received as a reception wave by use of the same ultrasonic element or a different ultrasonic element, and a period of time between transmission of the ultrasonic wave and reception of the reception wave is measured as a propagation time, the method being characterized by comprising: a reference-value setting step of integrating a reception wave or a portion thereof in order to obtain an integral value, and setting a reference value on the basis of the integrated value; and a time measurement step performed when a propagation time is to be measured and measuring, as an arrival time of the reception wave, a point in time when the level of a reception wave attains the reference value.
In embodiment (5), an ultrasonic reception wave or a portion thereof is integrated in order to obtain an integral value, and a reference value is set on the basis of the integrated value. At the time of actual measurement of propagation time, the point in time when the level of a reception wave attains the reference value is measured as an arrival time of the reception wave.
As described in relation to (1) above, ultrasonic reception waves sometimes are attenuated due to atmospheric pressure and other causes. Therefore, in embodiment (5), the reference value is adjustably set on the basis of an actual integral value. In particular, in embodiment (5), at the time of actual measurement, in place of an integral value of a reception wave, the reception wave itself is used for measurement of arrival time of the reception wave.
Like (1) above, embodiment (5) enables accurate measurement of propagation time. In addition, since a reception wave itself is used at the time of actual measurement, calculation and other processing performed during the measurement can be simplified.
(6) A gas sensor which detects the concentration of a specific gas in a gas under measurement by use of the ultrasonic-wave propagation-time measuring method according to any one of (1) to (5).
Propagation time of an ultrasonic wave varies depending on the concentration of a specific gas (to be detected), such as fuel vapor, contained in a gas under measurement, such as atmospheric air. Therefore, the concentration of a specific gas can be detected through measurement of propagation time of ultrasonic waves.
In particular, in embodiment (6), since propagation time can be measured accurately by use of the above-described ultrasonic-wave propagation-time measurement method, gas concentration can be detected accurately.
The expression xe2x80x9cintegrating a portion of a reception wavexe2x80x9d means integration of a portion (e.g., a portion useful for measurement) of a reception wave, not the entirety of the reception wave. Such integration provides the same effect as that obtained when the entirety of the reception wave is integrated. In some cases, integration of a portion of a reception wave is preferable, from the viewpoint of eliminating noise and decreasing the number of calculations.
(7) A gas-pressure measurement method comprising: transmitting an ultrasonic wave by use of an ultrasonic element; receiving a reflection wave of the transmission wave as a reception wave, by use of the same ultrasonic element or a different ultrasonic element; integrating a reception wave or a portion thereof in order to obtain an integral value; and determining the pressure of a gas under measurement on the basis of the integrated value.
In embodiment (7) above, the pressure of a gas under measurement is determined on the basis of the strength of an ultrasonic reception wave, making use of the relation between the strength of an ultrasonic reception wave and the pressure of a gas under measurement.
That is, when an ultrasonic wave propagates in a gas under measurement, the ultrasonic wave is attenuated to a greater degree as the pressure of the gas under measurement decreases, with the result that the strength of a reception wave becomes weak. Therefore, when an ultrasonic wave of constant strength is transmitted and the strength of a reception wave or a portion thereof having propagated through the gas under measurement is measured, the pressure of the gas under measurement can be measured on the basis of the strength.
In particular, in embodiment (7), a value obtained by integrating a reception wave or a portion thereof with respect to time is used as a parameter which represents the strength of the reception wave or a portion thereof. Use of such an integral value enables accurate measurement of the strength of the reception wave or a portion thereof, while eliminating influence of, for example, noise.
The following specific method may be used for calculating gas pressure from the above-mentioned integral value.
A map which represents the relationship between the integral value and pressure of the gas under measurement is prepared in advance, and the pressure of the gas under measurement is calculated using the map and an integral value obtained during actual measurement.
Unlike a method which utilizes a conventionally-employed diaphragm-type gas-pressure sensor, the gas-pressure measurement method according to the present invention does not require a mechanically-movable member. Therefore, the gas-pressure measurement method according to embodiment (7) is excellent in terms of durability and reliability of an apparatus.
(8) A gas-pressure measurement method comprising: transmitting an ultrasonic wave by use of an ultrasonic element; receiving a reflection wave of the transmission wave as a reception wave by use of the same ultrasonic element or a different ultrasonic element; and measuring the pressure of a gas under measurement on the basis of the maximum amplitude of the reception wave or a portion thereof.
As in embodiment (7), embodiment (8) provides a method of measuring the pressure of a gas under measurement while utilizing the strength of an ultrasonic reception wave. In particular, in embodiment (8), the maximum amplitude of a reception wave or a portion thereof is used as a parameter which represents the strength of the reception wave.
That is, as in the case of the integral value used in embodiment (7), the maximum amplitude is one parameter which indicates the strength of a reception wave and is a value which varies depending on the pressure of a gas under measurement. Therefore, the maximum amplitude can be used for gas-pressure measurement. For example, when the pressure of a gas under measurement is low, the maximum amplitude decreases.
Accordingly, as in the case of embodiment (7), embodiment (8) enables measurement of the pressure of a gas under measurement. In particular, in the present invention, the number of calculations during measurement can be reduced, because a process for integrating a reception wave is unnecessary.
(9) A gas-flow-rate measurement method which comprises: measuring the pressure of a gas under measurement by use of the gas-pressure measurement method described in (7) or (8); and measuring the flow rate of the gas under measurement on the basis of the measured pressure.
Embodiment (9) utilizes the phenomenon that the flow rate of a gas under measurement flowing through a flow path of a constant shape depends on a differential pressure produced in the flow path, and in the method of embodiment (9), a differential pressure produced in the flow path is first measured, and subsequently a gas flow rate is calculated on the basis of the differential pressure.
In order to measure the differential pressure in the flow path, a gas pressure at a predetermined location within the flow path is measured by use of the gas-pressure measurement method described in (7) or (8), and the differential pressure is obtained from the gas pressure.
In the case in which the gas pressure at the predetermined location in the flow path is constant (e.g., in the case in which one end of the flow path is exposed to the atmospheric pressure), the differential pressure in the flow path is univocally determined through measurement of the gas pressure at another point in the flow path. Therefore, measurement of gas pressure is required at only a single location.
Subsequently, the flow rate of the gas under measurement is calculated on the basis of the differential pressure obtained in the above-described manner.
Specifically, a map which represents the relationship between differential pressure produced in the flow path and flow rate of the gas under measurement is prepared in advance, and a flow rate of the gas under measurement is calculated using the map and an actually measured differential pressure.
In embodiment (9), since the pressure of a gas under measurement is measured, and the flow rate of the gas under measurement is calculated on the basis of the pressure, both the pressure and flow rate of the gas under measurement can be measured.
Therefore, when the pressure and flow rate of a gas under measurement are measured by use of embodiment (9), both the pressure and flow rate can be measured by use of a single gas sensor, so that cost and installation space of the gas sensor can be decreased.
(10) A gas sensor which comprises means for detecting the pressure of a gas under measurement using the gas-pressure measurement method described in (7) or (8).
The gas sensor of embodiment (10) provides effects similar to those described in relation to (7) and (8).
(11) A gas sensor which comprises means for detecting the concentration of a specific gas contained in a gas under measurement, by use of the ultrasonic-wave propagation-time measurement method described in any one of (1) to (5), and which comprises means for detecting the pressure of the gas under measurement by use of the gas-pressure measurement method described in (7) or (8).
Since the gas sensor of embodiment (11) measures the concentration of a gas under measurement in a manner similar to that used for the gas sensor described in (6), the gas sensor of embodiment (11) provides effects similar to those provided by the gas sensor described in (6).
Since the gas sensor of embodiment (11) measures the pressure of a gas under measurement in a manner similar to that used for the gas sensor described in (10), the gas sensor of embodiment (11) provides effects similar to those described in relation to (10).
Moreover, since embodiment (11) enables measurement of the concentration of a specific gas contained in a gas under measurement and the pressure of the gas under measurement by using a single gas sensor, cost and installation space of the gas sensor can be decreased as compared with the case in which gas concentration and gas pressure are measured by using different gas sensors.
Furthermore, the gas sensor of embodiment (11) measures gas concentration and gas pressure through utilization of attributes (propagation speed in measurement of concentration of a specific gas, and strength of a reception wave in pressure measurement) of ultrasonic waves propagating through the gas under measurement.
Accordingly, among structural portions of the gas sensor of embodiment (11), portions related to ultrasonic waves (e.g., an ultrasonic element, an ultrasonic propagation path, a reflection surface, and a signal processing circuit) can be used in common for gas-concentration measurement and gas-pressure measurement. Therefore, the gas sensor of embodiment (11) can be rendered further compact.
(12) A gas sensor which comprises means for detecting the pressure of a gas under measurement by use of the gas-pressure measurement method described in (7) or (8), and which comprises means for detecting the flow rate of the gas under measurement by use of the gas-flow-rate measurement method described in (9).
Since the gas sensor of embodiment (12) measures the concentration of a gas under measurement in a manner similar to that used for the gas sensor described in (6), the gas sensor of embodiment (12) provides effects similar to those provided by the gas sensor described in (6).
Further, since the gas sensor of embodiment (12) measures the flow rate of the gas under measurement in accordance with the gas-flow-rate measurement method described in (9), the gas sensor of embodiment (12) provides effects similar to those described in relation to (9).
Moreover, since embodiment (12) enables measurement of pressure and flow rate of a gas under measurement by use of a single gas sensor, cost and installation space of the gas sensor can be decreased.
In particular, the gas sensor of embodiment (12) calculates gas flow rate on the basis of a measured gas pressure, through use of, for example, a map. Therefore, hardware portions for measurement of gas flow can be reduced. Accordingly, the gas sensor of embodiment (12) can be made compact.
(13) A gas sensor which comprises means for detecting the concentration of a specific gas contained in a gas under measurement by use of the ultrasonic-wave propagation time measurement method described in any one of (1) to (5), means for detecting the pressure of the gas under measurement by use of the gas-pressure measurement method described in (7) or (8), and means for detecting the flow rate of the gas under measurement by use of the gas-flow-rate measurement method described in (9).
Since the gas sensor of embodiment (13) measures the concentration of a gas under measurement in a manner similar to that used for the gas sensor described in (6), the gas sensor of embodiment (13) provides effects similar to those provided by the gas sensor described in (6).
Since the gas sensor of embodiment (13) measures the pressure of the gas under measurement in a manner similar to that used for the gas sensor described in (10), the gas sensor of embodiment (13) provides effects similar to those provided by the gas sensor described in (10).
Moreover, since the gas sensor of embodiment (13) measures the flow rate of a gas under measurement in accordance with the gas-flow-rate measurement method described in (9), the gas sensor of embodiment (13) provides effects similar to those described in relation to (9).
Since embodiment (13) enables measurement of concentration of a specific component of a gas under measurement and pressure and flow rate of the gas under measurement by use of a single gas sensor, cost and installation space of the gas sensor can be decreased.
(14) The gas sensor described in (10), wherein the gas under measurement is a gas within an intake pipe or canister purge line in an internal combustion engine.
Since the gas sensor of embodiment (14) can measure gas pressure within the intake pipe or canister purge line, the gas sensor can be used for optimal control of the ratio between fuel and air supplied to the internal combustion engine.
The following example method may be employed for such control. The gas pressure within the intake pipe or canister purge line is measured by using the gas sensor of embodiment (13); the flow rate of the gas flowing within the intake pipe or canister purge line is calculated from the measured pressure (by the method described in (9)); and at the same time, the concentration of vaporized fuel in the gas is measured by use of another method. From the gas flow-rate and the concentration of vaporized fuel, the quantity of vaporized fuel supplied from the intake pipe to the internal combustion engine (hereinafter referred to as xe2x80x9cvaporized-fuel quantityxe2x80x9d) can be calculated.
Accordingly, the total quantity of fuel supplied to the internal combustion engine can be accurately calculated from the vaporized-fuel quantity and a known quantity of fuel supplied from an injector; and on the basis of the total quantity, the fuel/air ratio within a gas which takes part in combustion within the internal combustion engine can be controlled properly. As a result, toxic components contained in exhaust gas can be decreased in concentration.
(15) The gas sensor described in (11), wherein the gas under measurement is a gas within an intake pipe or canister purge line in an internal combustion engine; and a component of the gas under measurement is fuel for the internal combustion engine.
When a flow rate of a gas flowing within the intake pipe or canister purge line and the concentration of vaporized fuel within the gas are measured by using the gas sensor of embodiment (15), the quantity of vaporized fuel supplied from the intake pipe to the internal combustion engine can be calculated from the gas flow rate and the concentration of vaporized fuel.
Accordingly, as in embodiment (14), the fuel/air ratio within a gas which takes part in combustion within the internal combustion engine can be controlled properly, whereby toxic components contained in exhaust gas can be decreased in concentration.
(16) The gas sensor described in (12), wherein the gas under measurement is a gas within an intake pipe or canister purge line in an internal combustion engine.
When the flow rate of the gas flowing within the intake pipe or canister purge line is measured by using the gas sensor of embodiment (16), and the concentration of vaporized fuel in the gas is measured by use of another method, the quantity of vaporized fuel supplied from the intake pipe to the internal combustion engine can be calculated from the gas flow-rate and the concentration of vaporized fuel.
Accordingly, as in embodiment (14), the air/fuel ratio within a gas which takes part in combustion within the internal combustion engine can be controlled properly, whereby toxic components contained in exhaust gas can be decreased in concentration.
(17) The gas sensor described in (13), wherein the gas under measurement is a gas within an intake pipe or canister purge line in an internal combustion engine; and a component of the gas under measurement is fuel for the internal combustion engine.
When the flow rate of a gas flowing within the intake pipe or canister purge line and the concentration of vaporized fuel within the gas are measured by using the gas sensor of embodiment (17), the quantity of vaporized fuel supplied from the intake pipe to the internal combustion engine can be calculated from the gas flow rate and the concentration of vaporized fuel.
Accordingly, as in embodiment (14), the air/fuel ratio within a gas which takes part in combustion within the internal combustion engine can be controlled properly, whereby toxic components contained in exhaust gas can be decreased in concentration.